During the past several decades, ultrasonic imaging techniques have become increasingly prevalent in clinical diagnoses, and more particularly in obstetrics, gynecology and urology. Specialists in these disciplines use ultrasound to image a wide variety of medical abnormalities including malignant and non-malignant cysts and tumors and fetal status in utero as well as "real-time" monitoring of needle location during such procedures as fetal blood sampling, amniocentesis, tissue aspiration biopsy and core biopsy. Considerable effort has been expended to significantly enhance the ultrasound image of a needle, or at least its point or tip, in order to more accurately pinpoint its placement or advancement over real-time ultrasonic guidance. Not only is accurate guidance required to obtain the proper sample, but it is also necessary to avoid puncturing or damage to tissues.
The term echogenicity refers to the relative, intrinsic or innate degree or extent that a surface reflects incident ultrasound wave energy directly back to sensor, which is proximal to the source or emitter. The degree of echogenicity is directly interdependent on two primary factors, according to essential ultrasound physics: (1) the density of the "target" receiving and reflecting the sound energy, and (2) the elasticity of the "target" being ultrasonically imaged. These two factors are professed to be the essential reasons why air and/or water in tissue or organs are more "echogenic" or alter the echogenicity. The same applies to (dense) metal, such as the shaft of a needle.
Guess et al. U.S. Pat. No. 4,401,124 outlines some of the problems associated with monitoring the insertion and guidance of needles and other instruments. The Guess et al. patent also discloses a proposed solution to the monitoring problem by providing, in an ultrasound pulse-echo imaging system, a defraction granting disposed on the surface of the surgical instrument. The defraction grating is disclosed to have a specified distance D between the depth of adjacent grooves, that distance D being a function of various parameters including the center wavelength .lambda. of the transducer and the angle .theta. between the incident beam and a line along the surface of the instrument and perpendicular to the grooves. The Guess et al. reference also discloses other attempts directed toward monitoring the location of a surgical instrument, such as a needle, inside the body as well as discussing their drawbacks.
Although the Guess et al. system with its helical defraction grating around the tip of the needle, along with other needles having similar rings, may provide some degree of signal reinforcement along the axis of incident energy, the overall image is far from ideal. Further, needles of this type typically exhibit a marked loss of resolution as the needle is oriented away from an optimum angle relative to the incident ultrasound beam, which angle depends upon the particular ring parameters.
What is needed is a device which provides more accurate monitoring of a surgical instrument such as a needle inserted into the body, which does not require a specific angle of orientation for its efficiency, and which is inexpensive to manufacture.